Re: iSCSI: Iterating long text responses

[Santosh Rao <santoshr@cup.hp.com>]

Mark Bakke wrote:
> 
> Santosh-
> 
> I was making the assumption that a text command stayed on a single
> connection, and response PDUs from the target would be in-order,
> and that if the connection failed, the target would throw away any
> results it hadn't yet sent, and the initiator would just completely
> re-do the command. 


Mark,

The CRC error checking and the Text command processing may be handled in
different layers
of the iSCSI stack (CRC in hardware, text command in firmware or
software), with CRC errors causing a discard of the errant PDU alone.
Thus, upon receipt of the final Text Response PDU the layer handling the
Text command may not be aware that 1 PDU of the returned set of
responses was lost.

Some ways to address this would be :
a. usage of a RO in each Text Response PDU.
b. a count of the number of Text Response PDUs sent in the final Text
Response ("f=1"). 
c. A sequence count applied to all multi-PDU text sequences.

In addition, the spec needs to explicitly word the "connection
allegiance" description to apply to all iSCSI commands, and not just
SCSI commands.

> This meant we wouldn't need an offset, since
> the initiator would have to receive them in order anyway.

The offset is required to ensure integrity of the response received.

Regards,
Santosh


>  However,
> it certainly wouldn't hurt to add the offset.  I guess I am just
> concerned that if we do, then someone will want to make use of the
> field to be able to send text responses out of order, or to recover
> them on another connection, and so on, so I would rather leave them
> out unless we really need them.  How badly are they needed?
> 
> --
> Mark
> 
> Santosh Rao wrote:
> >
> > Mark,
> >
> > IMO, alternative #4 is sufficient and is semantically aligned with the
> > FC-GS-2/GS-3 GID_FT model.
> >
> > It does require modification of the Text Response PDU to provide a
> > Relative Offset (RO) field indicating the RO of the text response
> > PDU from the start of the text response payload, in order to allow initiators
> > to correctly re-assemble the received response PDUs.
> >
> > Regards,
> > Santosh
> >
> > Mark Bakke wrote:

> > >
> > > 4. Allow multiple response PDUs to a single text command:
> > >
> > >    I->T  Text Command (F bit set)
> > >    T->I  Text Response (first PDU, F bit cleared)
> > >    T->I  Text Response (next PDU, F bit cleared)
> > >
> > >    ...
> > >
> > >    T->I  Text Response (last PDU, F bit set)
> > >
> > >    Basically, we are doing (3) without the R2T.  The initiator,
> > >    upon sending the text command, must be prepared either to
> > >    accept as many PDUs as come back, or throw them away if it
> > >    can't handle them.  This solution is a lot like #1, but with
> > >    the response spread across 4k PDUs.
> > >
> > >    Also note that this (and the following scheme) avoid the problem
> > >    of the target keeping state; it sends ALL of the response PDUs
> > >    without the initiator asking for them.
> > >
> > > 5. Do #4, but allow the initiator to specify the amount of data
> > >    it is willing to accept:
> > >
> > >    I->T  Text Command (F bit set, AcceptResponseLength=4096)
> > >    T->I  Text Response (first PDU, F bit set, TotalResponseLength=12288)
> > >
> > >    In the above example, we have created a new text command field:
> > >
> > >       AcceptResponseLength
> > >
> > >    And in the text response PDU:
> > >
> > >       TotalResponseLength
> > >
> > >    The initiator indicated it was willing to accept a 4k response.
> > >    The target returned the first 4k in the text response, but set
> > >    the F bit since it was at its limit.  It also returned a
> > >    TotalResponseLength field.  Since this was greater than the
> > >    AcceptResponseLength, the initiator knows the amount of buffer
> > >    space it will need to get the full response.  It can then choose
> > >    whether it will re-send the command, and if so, can give it a
> > >    large enough response length:
> > >
> > >    I->T  Text Command (F bit set, AcceptResponseLength=12288)
> > >    T->I  Text Response (first PDU, F bit clear)
> > >    T->I  Text Response (next PDU, F bit clear)
> > >    T->I  Text Response (last PDU, F bit set, TotalResponseLength=12288)
> > >
> > >    Note that most initiators will probably send an AcceptResponseLength
> > >    of the largest response they would normally accept, and that most
> > >    target responses will fit in one or a few PDUs anyway.
> > >
> > >    #5 is really a compromise between #3 and #4; the target has the
> > >    benefit of being less statefull, and the initiator has the benefit
> > >    of controlling the amount of data it receives.
> > >
> > > I would like to recommend either #4 or #5.  I think that #5 is
> > > probably the safest solution, but #4 may not be a problem for anyone.
> > >
> > > Assuming that none of the implementors of initiators have a problem
> > > with #4, I would pick that.
> > >
> > > If they do have a problem with it, we should go with #5.
> > >
> > > Targets probably don't care much whether we do #4 or #5.
> > >
> > > Initiator developers-
> > >
> > >   Please indicate which solution (#4 or #5) appeals to you.
> > >
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